1. Field of Invention
The present invention relates to methods to manufacture a receiver, and more particularly to methods to manufacture a receiver suited to manufacture a GPS receiver or a cellular telephone having a GPS receiving function and to a receiver manufactured by using the same methods.
2. Description of Related Art
Recently, cellular telephones having GPS (Global Positioning System) receiving functions have been created along with the development of electronic technology. In the GPS functions, positioning signals are received and/or sent from three or more satellites at arbitrary locations of among nearly twenty four artificial satellites (GPS satellites) orbiting the earth every nearly twelve hours. Based upon the positioning signals, the distances to each satellite are determined to thereby determine a signal-receiving position from those distances.
The GPS satellites orbit the earth in a period of about twelve hours as noted above. Consequently, the positioning signal received from the GPS satellite at the terrestrial location fluctuates in frequency due to the Doppler effect. The amount of frequency fluctuation is different depending upon a relative velocity of the receiver and the GPS satellite. The fluctuation in the positioning signal from GPS satellite due to the Doppler effect can be determined by an operation based on the satellite orbit information previously stored by the GPS receiver and the location of the receiver. At the receiver, positioning is easy by making a correction to the fluctuation on the basis of a reference oscillator frequency. In the related art, as the GPS reference oscillator, a temperature compensated piezoelectric oscillator (Temperature Compensated Crystal Oscillator or TCXO) having a comparatively low fluctuation of oscillation frequency against ambient temperature change, is employed.
When receiving a GPS signal, there is a need to search the frequency ranges in which a satellite signal would be present until a satellite signal is received. Accordingly, in order to reduce the time from powering on the GPS receiver to the time that a satellite signal is captured, it is desirable to have a reference oscillator high in frequency accuracy. Ideally ±0.1 ppm or less is required. However, the usual TCXO has an oscillation frequency accuracy of nearly ±1.0 ppm to ±2.5 ppm at operating temperature range. In the case of using a TCXO, the frequency search range to capture positioning signals is broadened, requiring an increased time to capture a positioning signal. Moreover, the piezoelectric oscillator, such as TCXO, usually suffer from aging, thus raising a problem of increased errors.
In the related art, there is a proposal of a method of searching a positioning signal by the use of a usual piezoelectric oscillator, instead of a TCXO (Simple Packaged Crystal Oscillator: SPXO) including an AT-cut quartz resonator (hereinafter, AT resonator) as a reference oscillator, the frequency accuracy of reference oscillation frequency being substantially ±0.1 ppm or less (See e.g., JP-A-2001-281322). In the method described in JP-A-2001-281322, the curve representing a frequency-temperature characteristic of piezoelectric resonator is approximated as a quartic function having a variable of temperature, to detect an ambient temperature of the oscillator, whereby a frequency correction amount is determined depending upon the detected temperature and the function (approximate expression).
Generally, the SPXO using an AT-cut quartz resonator has a change rate of oscillation frequency in the usual operating temperature range of nearly four times or more that of the TCXO. Consequently, in the case of employing an SPXO as a reference oscillator for a GPS receiver, there is a necessity to measure the resonator for a frequency-temperature characteristic and to determine an approximate expression as noted above. In the related art, for the frequency-temperature characteristic, the GPS receiver maker, after purchasing a piezoelectric oscillator, measures frequency-temperature characteristics on the individual piezoelectric oscillators to thereby determine an approximate expression. Consequently, much labor and time is required to manufacture a GPS receiver.
Recently, there are widespread piezoelectric oscillators incorporating a piezoelectric resonator and an IC, such as an oscillator circuit, within one package. It can be considered that the characteristic data concerning a piezoelectric oscillator including the foregoing approximate expression and the coefficient of the approximate expression is stored in the IC memory incorporated within the piezoelectric oscillator package and provided to the receiver maker. However, in the case of adding a memory function to the oscillation IC, the IC increased in size proportional to the circuit addition related to data input/output and the number of bits for storage, thus making difficult to provide a piezoelectric oscillator satisfying the requirement of the client, such as cost and size-reduction requirement. Also, once the information concerning the frequency-temperature characteristic specific to a piezoelectric oscillator written in the memory within an oscillation IC is moved to a GPS receiver memory, from then on it is not necessary to use the memory within the oscillation IC, thus raising a drawback of much uselessness.